Since the immunologically compromised patient frequently develops pneumonias which are difficult to diagnose, aggressive diagnostic techniques are warranted. This study established a method of diagnosing unexplained pulmonary infiltrates utilizing selective bronchial brushing per oral fiberoptic bronchoscopy, or percutaneous lung biopsies when other methods fail. Forty patients with diffuse nodular infiltrates have undergone such procedures; of twelve patients not considered clinically infected, eight proved to have pulmonary metastases, three proved to have interstitial fibrosis, and one proved to have a collapsed lobe. Twenty-eight patients were clinically presumed infected and seventeen were proven to have pulmonary infection. Fourteen of the seventeen were etiologically documented by these procedures. Eleven of the twenty-seven patients had no etiology established by any method, although eight of these eleven patients were receiving broad spectrum antibiotics at the time of the procedure. The diagnoses included a variety of micro-organisms, both bacterial and fungal. Significant, but non fatal complications occurred and were acceptable risks for the patient population. There was no mortality associated with the procedures and despite pancytopenia, bleeding was not a problem.